1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system such as a copier, a printer, a facsimile machine, or a multifunction peripheral with two or more of copying, printing, and facsimile functions; in particular, relates to an image forming apparatus in which a transfer rotation body, which transfers a toner image borne by an image bearer such as a photoconductor drum or an intermediate transfer belt onto a recording medium, is arranged to abut the image bearer.
2. Description of the Related Art
Conventionally, in a technical field of an image forming apparatus such as a copier or a printer, in order to prevent a back surface or an edge surface of a recording medium which has been conveyed to a transfer nip unit from getting dirty due to toner which has moved from the image bearer and adhered to the transfer rotation body (transfer roller) at the transfer nip unit where the image bearer is pressed against the transfer rotation body, a technique is known in which a cleaning bias different from a transfer bias is applied to the transfer rotation body during time between sheets (between-sheets time)(refer to, for example, Patent Documents 1-3).
Here, during the between-sheets time, along with an image forming operation performed before and after the between-sheets time, a driving operation is continuously performed in which a driven member such as the image bearer is driven by a drive unit (drive motor).
On the other hand, Patent Document 1 discloses a technique in which, necessary time for cleaning (cleaning time) is calculated according to a size or a number of sheets of a recording medium which continuously passes through the transfer nip unit, the between-sheets time is increased or decreased according to the calculated cleaning time, and the cleaning mode of the predetermined time (cleaning of transfer rotation body) is performed.
Also, Patent Document 2 discloses a technique in which it is determined whether a cleaning mode should be performed according to the length of the between-sheets time which varies with various conditions during the continuous sheet passing, and, in the case where the between-sheets time has a certain length, the cleaning mode is performed from right after the beginning of the between-sheets time to a little before the end of the between-sheets time.
In the above technique disclosed in Patent Document 1, since the between-sheets time varies with cleaning time calculated according to predetermined conditions, there is a problem in which the between-sheets time may become too long and productivity during the continuous sheet passing may decrease.
Also, in the above technique disclosed in Patent Document 2, in the case where it is determined that the cleaning mode should be performed during the between-sheets time, since most of the between-sheets time is consumed by the cleaning mode, there is a problem in which degradation of the image bearer may be accelerated by direct contact with the transfer rotation body to which the cleaning bias is applied. Especially, in the case where the continuous sheet passing is frequently performed under a condition in which the between-sheets time is set long, the above problem becomes significant.
Also, in both techniques described above, since a driven member such as an image bearer is continuously driven by a drive unit, together with the image forming operation performed before and after the between-sheets time, during the between-sheets time in the continuous sheet passing, in the case where between-sheets time frequently becomes long, time driving the driven member becomes wastefully long and service life of the driven member or a related member may be shortened.
In order to solve the above problem, the present invention provides an image forming apparatus in which the degradation of the image bearer is not accelerated by a cleaning bias; productivity in the continuous sheet passing is not decreased by performing the cleaning mode; a problem, in which a back surface or an edge surface of a recording medium conveyed to the transfer nip unit may get dirty due to the toner moved from the image bearer and adhering to the transfer rotation body, is alleviated; and even in the case where between-sheets time frequently becomes long, the service life of the driven member may not be easily shortened.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2012-42641
[Patent Document 2] Japanese Laid-Open Patent Application No. 2003-140477
[Patent Document 3] Japanese Laid-Open Patent Application No. 2012-63497